The time constant of isovolumic relaxation (τ), and the transmitral Doppler E-wave deceleration time (DT) are typically used to characterize relaxation-related, diastolic left ventricular (LV) properties. DT may be assessed during routine studies or after a Valsalva maneuver for “pseudonormalized” E-wave patterns. When DT is prolonged and a Doppler E-wave peak velocity is lower than a Doppler A-wave peak velocity, the transmitral flow contour is said to possess a delayed relaxation pattern (DR). A DR pattern is an established hallmark of LV diastolic dysfunction. However, two LV chambers having indistinguishable τ and indistinguishable stiffness (ΔP/ΔV) can have significantly different DT. Furthermore, subjects with and without the DR pattern on echocardiography may have indistinguishable τ values. Thus, τ provides an incomplete characterization of relaxation during early-rapid filling. A mechanism for determining an invasive early-rapid filling index of LV relaxation/viscoelasticity that effectively correlates to the DR pattern is lacking in the art.